Impedance tuning circuit

ABSTRACT

A circuit for tuning an impedance matching network is disclosed. The circuit includes a current sensor, a control circuit coupled to the current sensor and a reference current source and a tunable capacitor coupled to the control circuit. The control circuit is configured to generate a control signal based on an output of the current sensor, wherein the control signal is configured to vary a capacitance of the tunable capacitor.

RELATED APPLICATIONS

This application is a divisional U.S. patent application Ser. No.14/645,177, filed on Mar. 11, 2015. This application is related toapplication Ser. No. 14/645,222, Attorney Docket No. 81672755US01entitled “Impedance Measurement” filed Mar. 11, 2015, and issued on May16, 2017 as U.S. Pat. No. 9,651,582 to this application and beingincorporated herein by reference. This application is also related toapplication Ser. No. 14/645,198, Attorney Docket No. 81672749US01entitled “Antenna Tuning Circuit” filed on Mar. 11, 2016, and issued onMay 9, 2017 as U.S. Pat. No. 9,647,706 to this application and beingincorporated herein by reference.

BACKGROUND

Small Loop antennas in printed circuit boards (PCB) are commonly used inapplications such wireless devices, for transmitting and receivingsignals. The impedance of the antenna is inductive and sensitive to theenvironmental changes. The environmental changes may cause non optimalsignal transmission operation. This mistuning causes power drop andlarge current variations in the power amplifier that provides the signalbeing transmitted via the antenna. The antennas are typically tuned atdesign or manufacturing stages and the devices containing these antennasare typically not configured to enable tuning of the antennas when thedevice is in use in the field.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

In some embodiments, the tunable capacitor includes a plurality ofswitchable capacitor circuits and the tunable capacitor is coupled tothe antenna. Each of the plurality of switchable capacitor circuitsincluding a capacitor coupled to a ground via a switch. The switch iscontrollable by the control signal. In one embodiment, the tunablecapacitor is coupled to an electrostatic discharge protection circuit.

In one or more embodiments, the control signal includes a plurality ofbits, each of the plurality of bits coupled to a different switchablecircuit in the plurality of switchable capacitor circuits. The controlsignal is an analog signal and the tunable capacitor is a voltagecontrolled variable capacitor.

In another embodiment, a system is disclosed. The system includes anantenna, an impedance matching circuit and an integrated circuitincluding a circuit for automatically tuning a variable capacitor basedon an impedance mismatch between the impedance matching circuit and theantenna.

In one embodiment, the antenna is fabricated on a printed circuit board.In another embodiment, the integrated circuit is mounted on the sameprinted circuit board. In some embodiments, the integrated circuit isconfigured to detect a current corresponding to the impedance mismatchbetween the impedance matching circuit and the antenna and generate abinary signal corresponding to the detected current. The binary signalis used to vary capacitance of the variable capacitor. The variablecapacitor includes a plurality of capacitors, each of the plurality ofcapacitors coupled to a ground through a switch. The switch is operablevia a bit of the binary signal.

In yet another embodiment, a tunable capacitor having a minimum and amaximum capacitance is disclosed. The tunable capacitor includes aplurality of capacitors, each of the plurality of capacitors having acapacitance substantially less than the maximum capacitance. Each of theplurality of capacitors is coupled to a ground through a separateswitching element. The tunable capacitor also includes an input port toreceive a binary signal wherein a bit of the binary signal being coupledto the switch of the each of the plurality of capacitors.

In one embodiment, the tunable capacitor also includes an electro staticprotection circuit. The tunable capacitor may also include a port toreceive a bias voltage. The switch is configured to be turn on or offbased on the value of the bit.

In one embodiment, the tunable capacitor further includes a minimumvalue capacitor of value equal to the minimum capacitance, wherein theminimum value capacitor is not coupled to the ground via a switch.

In yet another example, a circuit is disclosed. The circuit includes avoltage regulator, a current sensor coupled to the voltage regulator, avoltage sensor, a power amplifier coupled to the voltage sensor andcurrent sensor and a controller coupled to the current sensor and thevoltage sensor, wherein the controller is configured to produce acontrol signal based on inputs from the current sensor and the voltagesensor. In one example, the controller is coupled to current and voltagereferences and includes a comparator circuit that compares input fromthe current sensor and input from the voltage sensor to the current andvoltage references respectively, to produce the control signal. Inanother example, the controller is programmable and executed by aprocessor.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments. Advantages of the subject matter claimedwill become apparent to those skilled in the art upon reading thisdescription in conjunction with the accompanying drawings, in which likereference numerals have been used to designate like elements, and inwhich:

FIG. 1 is an a portion of a wireless communication system designed toprovide automatic antenna tuning;

FIG. 2 illustrates schematic of an integrated circuit depicted in FIG. 1in accordance to one of more embodiments of the present disclosure;

FIG. 3 illustrates a schematic of a tunable capacitor in accordance toone of more embodiments of the present disclosure; and

FIG. 4 illustrates a unit of the tunable capacitor shown in FIG. 3 inaccordance with an embodiment of the present disclosure.

FIG. 5 illustrates and impedance control system in accordance with oneor more embodiments of present disclosure.

DETAILED DESCRIPTION

FIG. 1 illustrates a portion of a wireless system including automaticantenna tuning 100. The wireless system 100 includes an integratedcircuit (IC) 110 that includes a circuit for automatically tuning anantenna 120 that is coupled to the IC 110. Typically but notnecessarily, the antenna 120 is printed or embossed on a printed circuitboard (PCB) and used in mobile devices, remote controls, keyless lockingsystems or similar applications in which a wireless communication isused. The wireless system 100 includes a matching network 130 that istypically a RLC or LC circuit or other combinations thereof, where R isresistance, L is inductance and C is capacitance.

Typically the matching network 130 is used to provide impedancematching. Typical loop antenna matching consists on capacitivetermination for the antenna 120 in both terminals so that a choice ofthe capacitors coupled to the antenna 120 brings the inductance of theantenna 120 to resonate to provide an equivalent parallel impedancetypically in the order of 50 ohms seen at the antenna input 121. Theother terminal of the antenna 120 is the antenna end 122. The IC 110includes various pins such as TX_OUT, VregOUT and ANT. TUNING (AntennaTuning Pin). The IC 110 may include other pins, such as a power pin, aground pin, etc. Other pins are not being shown so as not to obfuscatethe disclosure.

Mistuning of the antenna 120 may occur due to environmental factors. Thedevices that include the antenna 120 are typically designed such that itis not possible for the users of such devices to change the capacitanceat the antenna end 122 to correct any mistuning of the antenna 120.Besides, even if it can be made possible, it is unlikely that a typicaluser will be able to adjust the capacitance at the antenna end 122 foroptimal tuning.

Mistuning may typically be due to a change in inductance due toenvironment. The variation of the inductance can be compensated byadjusting the capacitors at the antenna 120 so that the equivalent seenat the antenna input still resonates like before without the mistuningeffect.

When a constant voltage but of varying frequency is applied to a circuitconsisting of an inductor, capacitor and resistor the reactance of boththe Capacitor/Resistor and Inductor/Resistor circuits is to change boththe amplitude and the phase of the output signal as compared to theinput signal due to the reactance of the components used. At highfrequencies the reactance of a capacitor is very low acting as a shortcircuit while the reactance of the inductor is high acting as an opencircuit. At low frequencies the reverse is true, the reactance of thecapacitor acts as an open circuit and the reactance of the inductor actsas a short circuit. Between these two extremes the combination of theinductor and capacitor produces a “Tuned” or “Resonant” circuit that hasa Resonant Frequency, in which the capacitive and inductive reactance'sare equal and cancel out each other, leaving only the resistance of thecircuit to oppose the flow of current. This means that there is no phaseshift as the current is in phase with the voltage.

Typically the values of capacitors C1 and C2A are calculated and fixedduring the design phase of a device that incorporate the antenna 120.ANT. TUNING pin is coupled to the antenna end 122 and the logic builtinto the IC 110 is configured to vary the capacitance at the antenna end122 to provide antenna tuning. Among others, one advantage of the IC 110is that in wireless applications, application designers can use the IC110 to provide “plug and play” automatic tuning of the antenna 120.

FIG. 2 illustrates the schematic of the IC 110. It should be noted thatonly some components of the IC 110 are being depicted in FIG. 2 so asnot to obfuscate the disclosure. The IC 110 includes a switched poweramplifier (PA) 190 coupled to TX_OUT pin. The switched PA 190 provides ahigh frequency signal to TX_OUT pin. A voltage regulator 180 provides alow frequency envelop signal and the voltage regulator 180 is coupled toVregOUT pin through a current sensor 160. Additionally, the switched PA190 may also be coupled to the voltage regulator 180. The current sensor160 is coupled to the voltage regulator 180 and a comparator and controllogic 170. The comparator and control logic 170 may be implemented insoftware or hardware. Typically a processor or a microcontroller (notshown) can provide the processing logic for comparing the current valuesreceived from the current sensor 160 and I_(ref) input.

In one embodiment, the comparator and control logic 170 is implementedusing a comparator that outputs a voltage corresponding to thedifference between I_(ref) and the current input from the current sensor160. The output of the comparator may be inputted to an analog todigital converter to obtain a binary output corresponding to the analogoutput from the comparator.

The output 200 of the comparator and control logic 170 is a coded word(a group of bits). In some embodiments the coded word may include aplurality of sub groups of bits. For example, one group may include bitsfor a first operation and the other group may include bits for a secondoperation. By the way of a non-limiting example, if 8 bit coded word isused, 4 bits may be used for coarse tuning of the antenna 102 and theremaining 4 bits may be used for fine tuning of the antenna 120.Alternatively, all bits of the coded word may be used for one type ofoperation such as tuning the antenna 120.

The IC 110 includes a tunable capacitor 150. The tunable capacitor 150is a composite component that provides variable capacitance that iscontrollable through the output 200 of the comparator and control logic170. The comparator and control logic 170 is programmed or configured togenerate the coded word based on the input from the current sensor 160and I_(ref). The output 200 may be a multibit bus for carrying a binarycode word. For example, in one embodiment, the output 200 may be a bussuitable for carrying at least 8 bit signal if the comparator andcontrol logic 170 provides 8 bit control signal.

FIG. 3 is a schematic of the tunable capacitor 150. The tunablecapacitor 150 includes a plurality of Ctunes 220. A Ctune is a componentthat includes a switchable capacitance. The number of Ctunes in thetunable capacitor 150 may be related to the width of the output 200. Forexample, if the output 200 is 8-bit wide, there may be 256 Ctunes in thetunable capacitor 150. The coded word in the output 200 may turn on oroff each of the Ctunes if the number of Ctunes corresponds to the widthof the coded word.

In some embodiments, the number of Ctunes may be less than that can becontrollable by the output 200 and if so, some codes in the output 200may be non-operational. Similarly, if the tunable capacitor 150 includesmore Ctunes than the number that can be digitally controlled by thecoded word in the output 200, the excess Ctunes may be used to provide aminimum capacitance.

The tunable capacitor 150 may also include an electrostatic discharge(ESD) protection circuit 230. ESD is a the sudden flow of electricitybetween two electrically charged objects due to various factors such aselectrical short, switching or dielectric breakdown. ESD can createspectacular electric sparks and cause permanent damage to a device. ESDprotection mechanisms are well known to a person skilled in the art,hence a detailed description is being omitted. In one embodiment, theESD protection 230 is designed for fast response time, low clamping andoperating voltages and for capacity to handle high peak ESD currents.The ESD protection 230 is also designed to remain undamaged byrepetitive ESD strikes and for a low reverse leakage current. In oneembodiment, the ESD protection 230 may be implemented usingsilicon-controlled rectifier and diodes.

As described above, the output 200 includes a binary word. Each of thebinary bits in the binary word is used to turn on or off a particularcapacitor/switch pair in the Ctune 220 in the tunable capacitor 150.Hence, a particular combination of zeros and ones in the binary word inthe output 200 can switch off (or on) some capacitors through turning ofrespective switches. Based on the logic built into the comparator andcontrol logic 170, the binary word includes appropriate number of zerosand ones to provide a desired capacitance in the tunable capacitor 150.

Further, as the capacitance of the tunable capacitor 150 changes, theimpedance of the antenna 120 also changes. This change in the antennaimpedance changes load and the current drawn from the voltage regulator180 also changes. Since the current values are continuously provided tothe comparator and control logic 170, in some embodiments, the processof tuning of the tunable capacitor 150 runs in a continuous loop.Therefore, the antenna 120 is automatically tuned automatically as soonas the environmental factors or external interferences cause a change inthe impedance of the antenna 120.

A V_(bias) generator 210 is used to provide a DC level that is highenough to prevent signal clipping effect. Typically, V_(bias) may besimply coupled to battery or a DC power source. If a V_(bias) voltagehigher than the battery voltage is required to prevent signal clipping,the V_(bias) generator 210 may include a circuit (e.g., a DC-DCconverter) to boost the DC power source voltage level.

The tunable capacitor 150 includes a port for receiving a bias voltageinput and a port for receiving a binary control signal. A bit of thebinary control signal is coupled to a switch of a Ctune 220. The tunablecapacitor 150 may also include a minimum value capacitor having a valuethat corresponds to the minimum tunable capacitance of the tunablecapacitor 150. This minimum value capacitor is not configured to beswitched on or off. In other words, this additional capacitor differsfrom the capacitors of a Ctune 220 in that the capacitor in the Ctune220 can be turned on or off while the same is not true for this minimumvalue capacitor. The tunable capacitor 150 has a maximum capacitance.The maximum capacitance can be derived when all capacitors in the Ctune220 are in the circuit. That is, the switches associated with each ofthe capacitors are on, thereby coupling the respective capacitors toground.

It should be noted that the output 200 may be an analog signal in someembodiment and the tunable capacitor 150 may include components that canchange capacitance based on an analog signal. For example, the tunablecapacitor 150 may include one or more varicap diodes that changecapacitance based on the applied voltage.

FIG. 4 depicts an example of a Ctune 220 circuit. It should be notedthat the voltage values indicated in FIG. 4 are provided merely asexamples. A Ctune 220 includes a capacitor 250 and a switch 260. Theswitch 260 is controllable by the output 200 of the comparator andcontrol logic 170. When the switch 260 is closed, the capacitor 250 isconnected to the ground and as a result, the capacitor 250 adds to thecapacitance to the tunable capacitor 150. If the switch 260 is open, thecapacitor 250 is dormant, hence, its values is not added to thecapacitance of the tunable capacitor 150. The comparator and controllogic 170 is calibrated and configured to produce a control signal(e.g., the output 200) to produce a desired capacitance via the tunablecapacitor 150.

In other embodiments, the tunable capacitor 150 can also be implementedsuch that the tunable capacitor 150 is tunable using analog means asopposed to digital words as described above. For example, the tunablecapacitor 150 may be a voltage controlled capacitor whose capacitancevaries based on the value of an analog input. In this example, thecomparator and control logic 170 will produce a variable voltage basedon the output of the current sensor 160 and I_(ref). In one embodiment,the comparator and control logic 170 is programmed or designed to keepthe current as sensed by the current sensor 160 within a preset bandbased on the value of I_(ref).

Referring back to FIG. 1, in some embodiments, the value of thecapacitor C2A is designed such that the value of the capacitor 250 isnot a large value because a large value of the capacitor 250 wouldrequire a large silicon area in the IC 110. On the other hand, to have awide tuning range, the value of the capacitor 250 should not be too low.In one embodiment, an optimal value of the capacitor C2A can becalculated based on experimental capacitance values needed for tuningthe antenna 120 under varying environmental change conditions.

An impedance of 50 ohms is a typical target used for the PA loadimpedance in the PA matching network. So the complete system is wellmatched when the antenna input is connected to the pa matching networkoutput. It should be noted that even though the disclosure uses anantenna tuning system to explain various circuits, a person of ordinaryskills will realize that the circuits described herein, especially thetunable capacitor circuit, may also be used in other applications thatrequire am automatically variable capacitance.

FIG. 5 illustrates an impedance control system 300 in another example.The impedance control system 300 includes an integrated circuit (IC)310. In addition to generating a control signal to alter the capacitanceof a variable capacitor, as described above, in this example, thecontroller 302 may be implemented to compare both a current input and avoltage input with respective reference values and generate a voltagesignal based on the both comparisons. In another example, the controller302 may be implemented in software that can be executed by a processor(not shown) and can produce a digital output corresponding to the inputvoltage and the input current. The output voltage signal may beconverted into a digital signal using a digital to analog converter. Inone example, the tunable matching circuit 316 may be implemented usingcomponents (such as varicap diodes) that can be operated through ananalog signal. In such examples, if the tunable matching circuit 316 iscontrollable by an analog voltage signal, the output of the controller302 does not need to be converted into a digital signal.

The control output is transmitted to the tunable matching circuit 316 ona control bus 314 that is adapted to be coupled to the tunable matchingcircuit 316. The tunable matching circuit 316 is coupled to an antenna318. It should be noted that even though an example of a receiver isused herein, the systems and methods described herein may also be usedfor tuning a matching network of a transmitter system without deviatingfrom the principles described herein.

In one example, a voltage regulator 312 applies a voltage to a poweramplifier 304. An impedance component 320 may be used in the coupling ofthe voltage regulator 312 and the power amplifier 304. Said couplingpath includes a current measuring device 308. In addition, a voltagemeasuring circuit or device 306 is used to measure voltage at the poweramplifier terminal that couples the power amplifier 304 to the tunablematching circuit 316. The voltage and current measurement are providedto the controller 302. In this example, higher values of current andvoltage would indicate a better tuning of the tunable matching network316.

It should be noted that impedance matching is the practice of designingthe input impedance of an electrical load or the output impedance of itscorresponding signal source to maximize the power transfer or minimizesignal reflection from the load. So as not to obfuscate the disclosure,many components that are known to a person skilled in the art are notbeing shown and/or discussed, at least in details.

Tuning the tunable matching circuit 316 based both on the current andvoltage inputs provides advantages of better tuning of the tunablematching circuit 316 for a complex load scenarios. For arbitraryimpedances, tuning based only on current consumption alone may notresult in optimum tuning. In one example, in addition to variablecapacitors, the tunable matching circuit 316 may also include variablecomponents of types other than variable capacitors. For example, thetunable matching circuit 316 may include variable resistor and/orvariable inductors, to effectively mimic a complex load. A tuning basedon the current input is performed by altering the value of one type ofvariable component such as the variable capacitor in the tunablematching circuit 316. It should be noted that in some examples, at leastsome variable components may be fabricated inside the IC 310.

After performing the tuning operation by altering one type of variablecomponent, the tuning is perform by altering another type of variablecomponent to achieve an optimal overall tuning for optimal power output.It should be noted that relationship between the power output to theimpedance of the components of the matching circuit 316 is not linear.Generally speaking, altering the value of a particular type of variablecomponent that contribute to the overall impedance of the tunablematching circuit 316 varies the output power on one axis. Morepractically, the variations follow roughly a circular path in on oneaxis. Altering the value of another type of impedance component (such asa resistor or inductor) varies the output power in another axis.Therefore, optimal power output can be achieved by tuning for bothcurrent and voltage.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the subject matter (particularly in the context ofthe following claims) are to be construed to cover both the singular andthe plural, unless otherwise indicated herein or clearly contradicted bycontext. Recitation of ranges of values herein are merely intended toserve as a shorthand method of referring individually to each separatevalue falling within the range, unless otherwise indicated herein, andeach separate value is incorporated into the specification as if it wereindividually recited herein. Furthermore, the foregoing description isfor the purpose of illustration only, and not for the purpose oflimitation, as the scope of protection sought is defined by the claimsas set forth hereinafter together with any equivalents thereof entitledto. The use of any and all examples, or exemplary language (e.g., “suchas”) provided herein, is intended merely to better illustrate thesubject matter and does not pose a limitation on the scope of thesubject matter unless otherwise claimed. The use of the term “based on”and other like phrases indicating a condition for bringing about aresult, both in the claims and in the written description, is notintended to foreclose any other conditions that bring about that result.No language in the specification should be construed as indicating anynon-claimed element as essential to the practice of the invention asclaimed.

Preferred embodiments are described herein, including the best modeknown to the inventor for carrying out the claimed subject matter. Ofcourse, variations of those preferred embodiments will become apparentto those of ordinary skill in the art upon reading the foregoingdescription. The inventor expects skilled artisans to employ suchvariations as appropriate, and the inventor intends for the claimedsubject matter to be practiced otherwise than as specifically describedherein. Accordingly, this claimed subject matter includes allmodifications and equivalents of the subject matter recited in theclaims appended hereto as permitted by applicable law. Moreover, anycombination of the above-described elements in all possible variationsthereof is encompassed unless otherwise indicated herein or otherwiseclearly contradicted by context.

1-11. (canceled)
 12. A system, comprising: an antenna; an impedance matching circuit; and an integrated circuit including a circuit for automatically tuning a variable capacitor based on an impedance mismatch between the impedance matching circuit and the antenna.
 13. The system of claim 12, wherein the antenna is fabricated on a printed circuit board.
 14. The system of claim 13, wherein the integrated circuit is mounted on the same printed circuit board.
 15. The system of claim 12, wherein the integrated circuit is configured to detect a current corresponding to the impedance mismatch between the impedance matching circuit and the antenna and generate a binary signal corresponding to the detected current.
 16. The system of claim 15, wherein the binary signal is used to vary capacitance of the variable capacitor.
 17. The system of claim 12, wherein the variable capacitor includes a plurality of capacitors, each of the plurality of capacitors coupled to a ground through a switch.
 18. A circuit, comprising: a voltage regulator; a current sensor coupled to the voltage regulator; a voltage sensor; a power amplifier coupled to the voltage sensor and current sensor; and a controller coupled to the current sensor and the voltage sensor, wherein the controller is configured to produce a control signal based on inputs from the current sensor and the voltage sensor.
 19. The circuit of claim 18, wherein the controller is coupled to current and voltage references and includes a comparator circuit that compares input from the current sensor and input from the voltage sensor to the current and voltage references respectively, to produce the control signal.
 20. The circuit of claim 18, wherein the controller is programmable and executed by a processor. 